The present invention relates to resin transfer molding processes and preforms useful in those processes.
Resin transfer molding processes are used to make fiber reinforced composite materials. Layers of reinforcing material are loaded into a mold, and a thermosetting resin is injected into the mold and cured using accepted procedures to provide a finished composite piece.
It is difficult and time consuming to load individual layers of fiber into the mold. Therefore, a thermoplastic, thermosetting resin is sometimes used as a "tackifier" to stiffen the reinforcing materials and hold them together before the molding process begins. See Heck et al., U.S. Pat. No. 4,992,228 (Feb. 12, 1991) and Flonc et al., U.S. Pat. No. 5,080,851 (Jan. 14, 1992), which are incorporated herein by reference. In such a process, the individual fiber plies are sprinkled with a solid powder of the tackifier. The individual ply, with the powder applied, is heated to above the melting point of the tackifier in order to fuse the tackifier to the surface of the substrate.
In a separate operation, the individual plies can be stacked together, heated and pressed to fuse the plies together, and then cooled to solidify the tackifier leaving a preform. The preform can be placed into the mold and used in an ordinary resin transfer molding process thereafter.